1. Field of the Invention
The present invention relates to an optical wavelength multiplexer/demultiplexer, and more particularly to an optical wavelength multiplexer/demultiplexer capable of suppressing a non-uniform loss between channels.
2. Description of the Related Art
The operation of an optical wavelength multiplexer/demultiplexer using an arrayed waveguide grating (AWG) can be defined by a grating equation describing dispersion characteristics of incident light resulting from a diffraction of the incident light under the condition in which an array of waveguides is regarded as a diffraction grating. Such an optical wavelength multiplexer/ demultiplexer is referred to as an AWG optical wavelength demultiplexer. Light incident to such an AWG optical wavelength demultiplexer varies in phase while passing through three parts of the AWG optical wavelength demultiplexer, that is, a first star coupler, an AWG, and a second star coupler. The phase variations of light respectively generated by the parts of the AWG optical wavelength demultiplexer are summed at a final output plane, namely, an interface between the AWG and the second star coupler, so that a reinforced interference effect is obtained at the final output plane. The above mentioned grating equation is an equation for deriving a condition in which a reinforced interference effect is obtained at the final output plane by virtue of the sum of the phase variations. Assuming that light is incident to a central to one of a plurality of input waveguides, the above mentioned grating equation is expressed as follow:
nsd sin xcex8+ncxcex94L=mxcexxe2x80x83xe2x80x83[Expression 1]
wherein xe2x80x9cnsxe2x80x9d represents effective refractive index of the star couplers, xe2x80x9cncxe2x80x9d represents an effective refractive index of the channel waveguides AWG, xe2x80x9cdxe2x80x9d represents the pitch of the AWG, xe2x80x9cmxe2x80x9d represents the order of diffraction, xe2x80x9cxcex94Lxe2x80x9d represents a length difference between adjacent waveguides in the AWG, and xe2x80x9cxcexxe2x80x9d represents the wavelength of incident light, respectively.
The central operating waivelength xcex0 corresponds to the wavelength of light when xe2x80x9cxcex8,xe2x80x9d in Expression 1, corresponds to zero. This central operating wavelength xcex0 is defined as follows:                               λ          0                =                                            n              C                        ⁢            Δ            ⁢                          xe2x80x83                        ⁢            L                    m                                    [                  Expression          ⁢                      xe2x80x83                    ⁢          2                ]            
From Expression 1, it is possible to derive an equation of a variation in the diffraction angle of light depending on the variation in wavelength. This equation can be expressed as follows:                                           ⅆ            θ                                ⅆ            λ                          =                  m                                    n              s                        ⁢            d                                              [                  Expession          ⁢                      xe2x80x83                    ⁢          3                ]            
As the wavelength of the incident light varies, as expressed in Expression 3, a variation in the wavefront direction of the light occurs. Such a variation in the wavefront direction of the light results in a variation in the main peak position of an interference pattern formed on the image plane of the second star coupler. That is, an interference pattern is formed at a position shifted in accordance with a variation in the wavelength of light wave. Accordingly, when an output waveguide is placed at the position where light of a desired wavelength is focused, a wavelength multiplexing/demultiplexing function can be conducted.
In the case of the above-mentioned AWG optical wavelength demultiplexer, however, a non-uniformity loss ranging from 2.5 dB to 3 dB is exhibited due to different diffraction efficiencies. In the case of an optical communication network having a large number of nodes, such non-uniformity of loss is accumulated as optical signals pass through each device, as mentioned above, so that its effect is amplified. This may serve as a severe limitation in configuring a desired system. In practice, one conventional method used to obtain uniform loss between the output channels is to use about a half of the total number of available output waveguides that can be coupled to the second slab waveguide. This can be realized by expanding the free spectral range (FSR) of the device by doubling the wavelength range to be used. Using this method, it is possible to reduce the non-uniformity loss within a range of 0.5 to 1 dB. However, this method reduces the number of devices that can be placed on a single wafer due to an increased size of the devices.
Meanwhile, the uniform gain uniformity can be improved by applying a plurality of AWG optical wavelength demultiplexers to compensate the waveguides with different losses. However, the above mentioned method requires many AWG optical wavelength demultiplexers to be cascaded in series, therefore, the loss non-uniformity of each device still remains.
In order to achieve an improvement in device, another method has been proposed by J. C. CHEN, et al (xe2x80x9cWAVEGUIDE GRATING ROUTERS WITH GREATER CHANNEL UNIFORMITYxe2x80x9d, Electronics Letters, 1997, vol. 33., no. 23, pp. 1951xcx9c1952). This method teaches an insertion of additional waveguides between adjacent waveguides in an AWG, as shown in FIG. 1.
The object of the invention is to provide an AWG optical wavelength emultiplexer of the type having a waveguide mode controller arranged between the AWG thereof and the second star coupler thereof, so that the second star coupler can form a flat diffraction pattern to obtain a uniformity of loss.
In accordance with the present invention, this object is accomplished by providing an optical wavelength multiplexer/demultiplexer for coupling or dividing optical signals of different wavelengths received from one or a plurality of input optical waveguides, and outputting the coupled or divided optical signals to one or a plurality of output optical waveguides, respectively, comprising: a first star coupler for dividing powers of the input optical signals received from the input optical waveguides; an arrayed waveguide grating for guiding the optical signals outputted from the first star coupler therethrough in such a fashion that the optical signals have a constant phase difference relative to the neighboring waveguides; a second star coupler for coupling or dividing the wavelengths of the optical signals outputted from the arrayed waveguide grating, and outputting the resultant optical signals to the output waveguides, respectively; and waveguide mode control means for controlling the profile of a waveguide mode of the optical signals outputted from the arrayed waveguide grating, thereby allowing the optical signals diffracted from each waveguide of the AWG have flat amplitude distributions at the image plane of the second star coupler.